CoCr2O4-BASED GAS SENSOR AND METHOD FOR MANUFACTURING THE SAME

ABSTRACT

A method of manufacturing a gas sensor for detecting xylene is provided. A method of manufacturing a gas sensor includes reacting a mixed material including a first material containing a cobalt (Co) element and a second material containing a chromium (Cr) element to form a CoCr2O4 hollow structure having a hollow shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

A claim for priority under 35 U.S.C. § 119 is made to Korean PatentApplication No. 10-2018-0171659 filed on Dec. 28, 2018, in the KoreanIntellectual Property Office, the entire contents of which are herebyincorporated by reference.

BACKGROUND

Embodiments of the inventive concept described herein relate to a gassensor for detecting xylene and a method of manufacturing a gas sensor.

As having excellent gas sensitivity and economical price, asemiconductor type gas sensor using oxide is portable and miniaturizedto be mounted on mobile and small devices and is advantageous to usewhere space is highly limited, such as a narrow indoor environment. Inaddition, the semiconductor type gas sensor may be widely used invarious applications such as industrial gas detection, driver drunknessmeasurement, food freshness measurement of a refrigerator, andenvironmental monitoring in a vehicle or indoors. Recently, as anindustry becomes high-tech and interest in human health andenvironmental pollution increases, there is a demand forhigh-performance gas sensitive materials to be used for an indoor andoutdoor environmental gas detection sensor, a self-diagnostic gas sensorfor diseases, and a high-performance artificial olfactory sensor whichis capable of being mounted on a mobile device. In particular, there isa need for an indoor pollution measuring sensor capable of preciselymonitoring environmental gases, which are generated in a room such as aliving space and an office space where many people have been active fora long time in their daily lives.

In particular, a volatile organic compound which is one of gasesnecessary to be detected is harmful to a human body and is difficult tograsp its existence because the volatile organic compound is colorlessand odorless and exists as gases at room temperature. The volatileorganic compound such as benzene, xylene, toluene, formaldehyde, andalcohol is continuously generated in furniture, paints, organicsolvents, paints, leather products, finishes, and the like, and it isknown that it is difficult to detect the volatile organic compound withhigh sensitivity using the oxide semiconductor gas sensor because thevolatile organic compound has a chemically stable macromolecularstructure. The volatile organic compound may cause various fataldiseases such as headache, dizziness, eye disease, skin disease, andcancer when the volatile organic compound is exposed to the human bodyfor a long time. Therefore, a gas sensitive material capable ofdetecting the volatile organic compound with high sensitivity is veryimportant. Most oxide semiconductor gas sensors have a problem in thatthey exhibit similar sensitivity to the above-described gases or exhibithigh reactivity to alcohol and formaldehyde, which frequently occur inindoor environments. However, the recommended lowest concentration limitfor each volatile organic compound is different. Furthermore, benzene isknown to be a carcinogen and xylene and toluene may cause variousdiseases of the respiratory and nervous systems. Therefore, there is aneed for a gas sensor having selective sensitivity because each gas hasa different effect on the human body and a manifested disease, asdescribed above.

Many methods have been proposed to produce a gas sensitive materialcapable of being selectively detected by adding and applying a catalystsuch as a heterooxide, a noble metal, and the like, having excellentcatalyst activation to a specific gas, to an oxide semiconductor.Additional processes have been proposed, such as attachment of aspecific gas filter to a gas sensor for increasing selectivity. However,these methods have a problem in that a degree of activation of singlegas selectivity is insignificant or an additional cost for the processaddition is increased and optimization and quantification of thecatalyst is difficult. In particular, when selectivity to a hindered gasthrough the above methods is imparted, selective detection between gaseshaving a benzene ring and having a similar molecular structure, such asxylene, toluene and benzene, is difficult.

SUMMARY

Embodiments of the inventive concept provide a gas sensor having highselectivity and high sensitivity to xylene and a method of manufacturingthe gas sensor.

The problem to be solved by the inventive concept is not limitedthereto, and other problems not mentioned will be clearly understood bythose skilled in the art from the following description.

According to an exemplary embodiment, a method of manufacturing a gassensor for detecting xylene includes reacting a mixed material includinga first material containing a cobalt (Co) element and a second materialcontaining a chromium (Cr) element to form a CoCr₂O₄ hollow structurehaving a hollow shape.

The mixed material may further include citric acid.

The first material and the second material may be provided to the mixedmaterial to be a molar ratio between the cobalt element and the chromiumelement of 1:2 to 1:4.

The mixed material may further include a noble metal catalyst and thenoble metal catalyst may include Pt, Pd, or Au.

The first material may include cobalt (II) nitrate hexahydrate(Co(NO₃)₂.6H₂O) and the second material may include chromium (III)nitrate nonahydrate (Cr(NO₃)₃.9H₂O).

The forming the hollow structure may include dissolving the mixedmaterial in distilled water to prepare a spray solution; spraying thespray solution and heating the sprayed spray solution to form a CoCr₂O₄precursor; and performing heat treatment of the CoCr₂O₄ precursor.

The method may further include coating the CoCr₂O₄ hollow structureprepared in the forming of the hollow structure on an insulatorsubstrate where an electrode is provided.

According to an exemplary embodiment, a gas sensor includes a sensitivelayer sensitive to xylene wherein the sensitive layer includes a CoCr₂O₄hollow structure.

The sensitive layer may further include Cr₂O₃.

The sensitive layer may further include a noble metal catalyst.

The gas sensor may further include an insulator substrate formed of aninsulator material; and an electrode connected to the insulatorsubstrate, wherein the sensitive layer may be coated on the insulatorsubstrate, and the electrode may be connected between the insulatorsubstrate and the sensitive layer.

The gas sensor may further include a heater heating the sensitive layer.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects and features will become apparent from thefollowing description with reference to the following figures, whereinlike reference numerals refer to like parts throughout the variousfigures unless otherwise specified, and wherein:

FIG. 1 is a cross-sectional view illustrating a gas sensor according toan embodiment of the inventive concept;

FIG. 2 is a flow chart illustrating a method of manufacturing a gassensor according to an embodiment of the inventive concept;

FIG. 3 is a graph illustrating results of X-ray diffraction analysis ofExamples 1-1, 1-2, 2-1, 2-2, 2-3, and 2-4 and Comparative Examples 1-1and 1-2;

FIG. 4 is SEM and TEM pictures taken secondary particle structures offine powders prepared in each Example and each Comparative Example;

FIG. 5 is a graph illustrating gas sensitivity to ethanol, xylene,toluene, benzene, formaldehyde, trimethylamine, ammonia and carbonmonoxide, each which had 5 ppm concentration at an operating temperatureof 275° C. in Example 1-1 and Example 2-1, Comparative Example 1-1, andComparative Example 1-2;

FIG. 6 is a graph illustrating gas sensitivity for ethanol, xylene,toluene, benzene, formaldehyde, trimethylamine, ammonia and carbonmonoxide, each which had 5 ppm concentration at an operating temperatureof 275° C. in gas sensors manufactured by Examples 1-2, 1-3, 2-2, 2-3,2-4;

FIG. 7 is a graph illustrating changes of sensitivity ratios of xylenesensitivity to ethanol sensitivity of gas sensors manufactured inExamples 1-1, 1-2, 1-3, and 2-1 and Comparative Examples 1-1 and 1-2with temperature;

FIG. 8 is a graph illustrating selectivity (S_(X)/S_(E)) of xylene inExample 2-2, Example 2-3, and Example 2-4 at an operating temperature275° C.;

FIG. 9 is a graph illustrating sensitivity characteristics of xylene gasin Example 1-2 (300° C.) and Example 2-2 (275° C.) at sensor operatingtemperatures showing the highest selectivity with respect to changes ofgas concentration;

FIG. 10 is a graph illustrating results of comparing xylene sensitivityfor each concentration with the existing studies; and

FIG. 11 is a graph showing results of measuring gas sensitivity (R_(g)R_(a) ⁻¹) of Example 1-2 of the inventive concept for 30 days.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the inventive concept will be described inmore detail with reference to the accompanying drawings. The embodimentsof the inventive concept may be modified in various forms, and the scopeof the inventive concept should not be construed as being limited to thefollowing embodiments. This embodiment is provided to more completelyexplain the inventive concept to those skilled in the art. Therefore,the shape of the elements in the drawings is exaggerated to emphasize amore clear description.

The inventive concept provides a highly sensitive and selective oxidesemiconductor gas sensor having a high selectivity to xylene using pureCoCr₂O₄ while having a very low gas sensitivity to a hindered gas suchas benzene, formaldehyde, alcohol, and the like. In addition, theinventive concept may provide a gas sensor where various catalysts (Pt,Au, or the like) are added to increase the sensitivity and selectivityto xylene through a synergistic effect of the catalysts and CoCr₂O₄.

FIG. 1 is a cross-sectional view illustrating a gas sensor 10 accordingto an embodiment of the inventive concept. Referring to FIG. 1, the gassensor 10 includes a sensitive layer 100, an insulator substrate 200,electrodes 301 and 302, and a heater 400. The gas sensor 10 detectsxylene.

The sensitive layer 100 is sensitive to xylene. For example, when thesensitive layer 100 is in contact with xylene, an electrical resistancechanges. The sensitive layer 100 includes a CoCr₂O₄ hollow structure.According to an embodiment, the sensitive layer 100 may further includeCr₂O₃. Alternatively, the sensitive layer 100 may further include anoble metal catalyst. For example, Pt, Pd, Au, or Rh may be provided asthe noble metal catalyst.

The insulator substrate 200 is provided to electrically connect theelectrodes 301 and 302 to the sensitive layer 100. The insulatorsubstrate 200 is provided as an insulator. For example, alumina (Al2O3)may be provided as the insulator substrate 200.

The electrodes 301 and 302 are connected to a top surface of theinsulator substrate 200. The sensitive layer 100 is coated on theinsulator substrate 200 whose the top surface is connected to theelectrodes 301 and 302. Accordingly, the electrodes 301 and 302 areconnected between the insulator substrate 200 and the sensitive layer100. A resistance measuring device for measuring an electricalresistance is connected to the electrodes 301 and 302 which areconnected to the insulator substrate 200 and the sensitive layer 100.When the sensitive layer 100 is in contact with xylene, the electricalresistance of the sensitive layer 100 may be changed and the changedelectrical resistance of the sensitive layer 100 may be measured by theresistance measuring device to detect xylene.

The heater 400 heats the sensitive layer 100 to a temperature at whichthe sensitive layer 100 is activated for detection of xylene. Accordingto an embodiment, the heater 400 may be provided on a bottom surface ofthe insulator substrate 200. The heater 400 may include a heating wirewhich generates heat by the electrical resistance. According to anembodiment, the heater 400 may heat the sensitive layer 100 to 250 to350° C.

Hereinafter, a method of manufacturing a gas sensor according to anembodiment of the inventive concept will be described. The gas sensor 10of FIG. 1 is an example of a gas sensor manufactured by the method ofmanufacturing the gas sensor according to an embodiment of the inventiveconcept.

Pure Co₃O₄ (Comparative Example 1-1) and pure Cr₂O₃ (Comparative Example1-2) were synthesized using a spray pyrolysis method as ComparativeExamples for Examples of the inventive concept. Pure CoCr₂O₄ sensitivematerials (Example 1-1, Example 2-1) were synthesized through adjustingaddition of precursors of Co and Cr using the same method. It wasconfirmed that pure CoCr₂O₄ had a composition advantages for xylene gassensitization through experimental data presented below, unlike pureCo₃O₄ and pure Cr₂O₃.

In addition, the CoCr₂O₄ sensitive materials (Example 1-2, Example 1-3)which were produced with Cr₂O₃ in the same manner and CoCr₂O₄ sensitivematerials added with Pt, Pd, or Au (Examples 2-2, 2-3, 2-4) weresynthesized and gas sensitive properties were checked. In particular,the addition of the Pt catalyst greatly improved sensitivity to xylene,while lowering sensitivity to indoor environmental gases such asbenzene, ethanol, formaldehyde, carbon monoxide, and the like, therebyincreasing the detection selectivity of xylene. Accordingly, theembodiment of the inventive concept for sensitively and selectivelydetecting the specific gas is not limited to a manufacturing methodhaving only CoCr₂O₄ content, includes a nanocomposite and a solidmixture containing Cr and Co of various compositions, and includes thesensitive material in which a noble metal catalyst, such as Pd or Au, isadded to CoCr₂O₄.

In the inventive concept, pure Co₃O₄ (Comparative Example 1-1), pureCr₂O₃ (Comparative Example 1-2), pure CoCr₂O₄ (Example 1-1, Example 2-1)and Cr₂O₃—CoCr₂O₄ (Example 1-2, Example 1-3), Pt—CoCr₂O₄ (Example 2-2),Pd—CoCr₂O₄ (Example 2-3), Au—CoCr₂O₄ (Example 2-4) were synthesizedusing the spray pyrolysis method to manufacture gas sensors,respectively. After manufacturing each gas sensor using each preparedsensitive material, the gas sensitivity of ethanol, xylene, toluene andthe like were compared. In addition, the selectivity of xylene relativeto ethanol was measured for all the Comparative Examples and Examplesand the lowest limits of xylene detection of Examples 1-2 and Example2-2 were measured.

As described above, a catalytic reaction between CoCr₂O₄ and Cr₂O₃,which are effective for decomposition of xylene, and Pt reduces the gassensitivity of the oxide semiconductor gas sensor to ethanol orformaldehyde, which is highly reactive while the gas sensitivity toxylene known for being weak reactivity is significantly increased to becapable of selectively sensitive to xylene, i.e., an indoorenvironmental gas.

FIG. 2 is a flow chart illustrating a method of manufacturing a gassensor according to an embodiment of the inventive concept. Referring toFIGS. 1 and 2, a method of manufacturing a gas sensor according to anembodiment of the inventive concept manufactures a gas sensor fordetecting xylene. The method of manufacturing the gas sensor includesforming a hollow structure in S10 and performing coating in S14.

In the forming of the hollow structure in S10, a mixed materialincluding a first material containing a cobalt (Co) element, a secondmaterial containing a chromium (Cr) element, and citric acid is reactedto form the CoCr₂O₄ hollow structure to have a hollow shape.

The first material and the second material are provided to the mixedmaterial such that a molar ratio between the cobalt element and thechromium element is 1:2 to 1:4. According to an embodiment, cobalt (II)nitrate hexahydrate (Co(NO₃)₂.6H₂O) may be provided as the firstmaterial and chromium (III) nitrate nonahydrate (Cr(NO₃)₃.9H₂O) may beprovided as the second material.

The mixed material may further include a noble metal catalyst. Forexample, Pt, Pd, Au, or Rh may be provided as the noble metal catalyst.

According to an embodiment, the forming of the hollow structure in S10includes preparing a spray solution in S11, heating spray in S12, andperforming heat treatment in S13.

According to an embodiment, in the preparing of the spray solution inS11, the mixed material is dissolved in distilled water to prepare thespray solution.

In the heating of spray in S12, the spray solution prepared in theforming of the spray solution in S11 is sprayed and the sprayed spraysolution is heated to form a CoCr₂O₄ precursor.

In the performing of the heat treatment in S13, the CoCr₂O₄ precursorprepared in the heating of spray in S12 is heat-treated.

In the performing of the coating in S14, the CoCr₂O₄ hollow structureprepared in the forming of the hollow structure in S10 is coated on theinsulator substrate 200 where the electrodes 301 and 302 are provided.

Hereinafter, detailed Examples manufacturing gas sensors using theabove-described method of manufacturing the gas sensor and effects ofthe gas sensors manufactured according to each embodiment will bedescribed.

Example 1-1

According to Example 1-1, in the preparing the spray solution in S11, amolar ratio of Cr/Co was calculated to be 2.0 in distilled water of 200mL and cobalt (II) nitrate hexahydrate [Co(NO₃)₂.6H₂O, 99.999%,Sigma-Aldrich, US] of 1.5 g, chromium (III) nitrate nonahydrate[Cr(NO₃)₃.9H₂O, 99%, Sigma-Aldrich, US] of 4.1 g, and citric acid[C₆H₈O₇, 99.5%, Sigma-Aldrich, USA] of 4.2 g were mixed and stirreduntil all the reagents are dissolved to prepare the spray solution.

In the heating of the spray in S12, the spray solution prepared in thepreparing the spray solution in S11 was ultrasonically sprayed in air ata flow rate of 10 L min⁻¹ and simultaneously passed through an electricfurnace (600° C.) connected to a spray outlet to form the pure CoCr₂O₄hollow structure precursor.

In the performing of the heat treatment in S13, the CoCr₂O₄ precursorformed in the heating of the spray in S12 was heat-treated for 2 hoursat 700° C. to form the CoCr₂O₄ hollow structure having the hollow shape.

In the performing of the coating step in S14, fine powders of theCoCr₂O₄ hollow structure in the performing of the heat treatment in S13were mixed with distilled water, were dropped onto an alumina (Al₂O₃)substrate in which an Au electrode was formed to be coated, andperformed the heat treatment at 450° C. for 2 hours to manufacture thegas sensor.

A method of measuring gas sensitivity of the manufactured gas sensor isas follows.

The manufactured gas sensor was placed inside a gas sensing chamberhaving a quartz tube, pure air or mixed gas was alternately injected,and the resistance change of the gas sensor was measured in real time.Gases were mixed at an appropriate concentration in advance through amass flow controller (MFC), and then rapidly injected using a 4-wayvalve to change the gas concentration inside the gas sensing chamber.The total flow rate inside the gas sensing chamber was fixed at 200 SCCMto maintain a temperature of the gas sensor in spite of a sudden changein gas concentration.

Example 1-21

According to Example 1-2, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 3.0 in distilled water of200 mL to synthesize the CoCr₂O₄ hollow structure with preparing Cr₂O₃and cobalt (II) nitrate hexahydrate of 1.5 g, chromium (III) nitratenonahydrate of 6.1 g, and citric acid of 4.2 g were mixed and stirreduntil all the reagents are dissolved to prepare the spray solution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

Example 1-31

According to Example 1-3, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 4.0 higher than Example 1-2in distilled water of 200 mL and cobalt (II) nitrate hexahydrate of 1.5g, chromium (III) nitrate nonahydrate of 8.2 g, and citric acid of 4.2 gwere mixed and stirred until all the reagents are dissolved to preparethe spray solution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

Example 2-11

According to Example 2-1, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 2.0 in distilled water of200 mL and cobalt (II) nitrate hexahydrate of 0.38 g, chromium (III)nitrate nonahydrate of 1.02 g, and citric acid of 1.0 g were mixed andstirred until all the reagents are dissolved to prepare the spraysolution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

Example 2-2

According to Example 2-2, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 2.0 in distilled water of200 mL to synthesize Pt-added CoCr₂O₄ hollow structure and cobalt (II)nitrate hexahydrate of 0.38 g, chromium (III) nitrate nonahydrate of1.02 g, chloroplatinic acid solution (8 wt % in H₂O, H₂PtCl₆,Sigma-Aldrich, USA) of 0.05 mL, and citric acid of 1.0 g were mixed andstirred until all the reagents are dissolved to prepare the spraysolution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

Example 2-3

According to Example 2-3, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 2.0 in distilled water of200 mL to synthesize Pd-added CoCr₂O₄ hollow structure and cobalt (II)nitrate hexahydrate of 0.38 g, chromium (III) nitrate nonahydrate of1.02 g, palladium nitrate hydrate (Pd(NO₃)₂.xH₂O, Sigma-Aldrich, USA) of0.0014 g, and citric acid of 1.0 g were mixed and stirred until all thereagents are dissolved to prepare the spray solution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

Example 2-4

According to Example 2-4, in the preparing of the spray solution in S11,a molar ratio of Cr/Co was calculated to be 2.0 in distilled water of200 mL to synthesize Au-added CoCr₂O₄ hollow structure and cobalt (II)nitrate hexahydrate of 0.38 g, chromium (III) nitrate nonahydrate of1.02 g, gold chloride trihydrate (HAuCl₄.3H₂O, Sigma-Aldrich, USA) of0.0012 g, and citric acid of 1.0 g were mixed and stirred until all thereagents are dissolved to prepare the spray solution.

Thereafter, the method of manufacturing the gas sensor and the method ofmeasuring the gas sensitivity were the same as in Example 1-1.

The gas sensors were prepared using the fine powders synthesized in theabove Examples and Comparative Examples and were measured at varioustemperatures, to exhibit p-type semiconductor type characteristics inwhich resistances were increased for all measured reducing gases.Therefore, gas sensitivity was defined as R_(g)R_(a) ⁻¹ (R_(g): deviceresistance in gas, R_(a): device resistance in air). The gas sensorswere manufactured using the synthesized fine powders, and then the gassensitivity was measured, and the selectivity was calculated based onthe difference in sensitivity from other gases.

When the resistance of each sensor stabilized in the air, the atmospherewas changed to test gas (ethanol, xylene, toluene, benzene,formaldehyde, trimethylamine, ammonia, carbon monoxide each of 5 ppm),and when the resistance in the test gas was constant, the atmosphere waschanged back to the air to measure the resistance change. A finalresistance reached when exposed to the test gas was defined as R_(g) anda resistance in the air was defined as R_(a). The xylene selectivitymeasured by each gas sensor was calculated from a ratio (S_(X)/S_(E)) ofxylene sensitivity “S_(X)” to ethanol sensitivity S_(E)”. Here, ethanolis the hindered gas.

FIG. 3 is a graph illustrating results of X-ray diffraction analysis ofExamples 1-1, 1-2, 2-1, 2-2, 2-3, and 2-4 and Comparative Examples 1-1and 1-2. Referring to FIG. 3, it was confirmed that Example 1-1 andExample 2-1 included CoCr₂O₄ without Co₃O₄ or Cr₂O₃ diffraction patternsthrough the results of the X-ray diffraction analysis, same asComparative Examples 1-1 and 1-2. It was confirmed that Example 1-2 hada pattern of a nanocomposite in which Cr₂O₃ and CoCr₂O₄ are mixed at aspecific ratio and Examples 2-2, 2-3, and 2-4 showed CoCr₂O₄ diffractionpatterns. The diffraction patterns of the noble metal catalysts (Pt, Pd,and Au) in Examples 2-2, 2-3 and 2-4 were not confirmed due to detectionlimit of the X-ray diffraction analysis.

FIG. 4 is SEM and TEM pictures taken secondary particle structures offine powders prepared in each Example and each Comparative Example.Referring to FIG. 4, it was confirmed that the particles prepared in allExamples and Comparative Examples had a spherical hollow structure.

FIG. 5 is a graph illustrating gas sensitivity to ethanol, xylene,toluene, benzene, formaldehyde, trimethylamine, ammonia and carbonmonoxide, each which had 5 ppm concentration at an operating temperatureof 275° C. in Example 1-1, Example 2-1, Comparative Example 1-1, andComparative Example 1-2. Referring to FIG. 5, Comparative Examples 1-1and 1-2, which were hollow structures synthesized only with Co₃O₄ andCr₂O₃ respectively, were confirmed to have little sensitivity to theabove-described gases. It was confirmed that pure CoCr₂O₄ showedexcellent sensitivity to xylene (S_(X)=61.4) through evaluation of gassensitivity characteristics of Example 1-1. In addition, it wasconfirmed that the gas sensitivity of Example 2-1, which was pureCoCr₂O₄ having a thin shell by reducing addition amounts of Co and Cr,was greatly increased (S_(X)=319.5). This showed that CoCr₂O₄ exhibitedhigh sensitivity and selectivity to xylene, regardless of its structure,and the sensitive material having the thin shell rapidly spreads the gasintroduced into the gas sensitive material to the gas sensitive materialinside, thereby increasing the gas sensitivity.

FIG. 6 is a graph illustrating gas sensitivity for ethanol, xylene,toluene, benzene, formaldehyde, trimethylamine, ammonia and carbonmonoxide, each which had 5 ppm concentration at an operating temperatureof 275° C. in gas sensors manufactured by Examples 1-2, 1-3, 2-2, 2-3,and 2-4. Referring to FIG. 6, it was confirmed that the sensitivity toxylene increased when a specific amount of Cr₂O₃ secondary phase wasgenerated through Examples 1-2 and 1-3 but when the Cr₂O₃ secondaryphase was excessively generated, the gas sensitivity was low for allgases. Therefore, it could be predicted that the gas sensitivity was lowfor all gases when the molar ratio of Cr/Co was 4.0 or more. The xylenesensitivity of Example 1-2 was greater than the xylene sensitivity ofExample 1-1. This was because transfer of charge between two p-typesemiconductors having different work functions occurs to generateelectrical sensitization of the CoCr₂O₄ sensitive material when theCrC2O4 phase was predominant and Cr₂O₃ was discontinuously added.Furthermore, the addition of the noble metal catalysts Pt and Au throughExamples 2-2 and 2-4 greatly increased the xylene sensitivity. However,it was thought that the addition of Au also increased the sensitivity ofethanol, i.e., the hindered gas, by the electronic sensitization to havethe xylene selectivity slightly lower than that of the addition of Pt.It was shown that Pd-added CoCr₂O₄ of Example 2-3 had the xylenesensitivity (S_(X)=225.8) lower than that of pure CoCr₂O₄ (S_(X)=319.5).It was thought to be due to oxidation of xylene at an upper end of asensor sensitive layer by catalytic activity of Pd, which was known asan oxidation catalyst for methylbenzene, such as xylene and toluene, andCoCr₂O₄. Therefore, it could be confirmed that the Pt catalyst served asgreatly improving the sensitivity and selectivity to xylene in theCoCr₂O₄ sensor.

FIG. 7 is a graph illustrating changes of sensitivity ratios of xylenesensitivity to ethanol sensitivity of gas sensors manufactured inExamples 1-1, 1-2, 1-3, and 2-1 and Comparative Examples 1-1 and 1-2with temperature. Referring to FIG. 7, Comparative Example 1-1 andComparative Example 1-2 were shown that it was impossible to selectivelydetect xylene using Co₃O₄ and Cr₂O₃ because the selectivity(S_(X)/S_(E)) of the gas sensor with respect to xylene had a valuewithin 1 at all temperatures. Example 1-1 and Example 2-1 showed valuesof S_(X)/S_(E)=12.3 and 30.7 at 275° C., respectively and thepossibility of highly selective detection of xylene with CoCr₂O₄ alone.It was shown that the sensitivity to xylene gas increased to increasethe selectivity of xylene when a certain amount of Cr₂O₃ secondary phasewas generated through Example 1-2. Furthermore, it was confirmed that,when the amount of Cr₂O₃ secondary phase excessively increased (Examples1-3, the molar ratio of Cr/Cr was 4.0), the low xylene selectivitysimilar to Comparative Example 1-2 i.e., pure Cr₂O₃, was exhibitedbecause the gas sensitivity occurred mainly through Cr₂O₃ having a lowsensitivity. The above results showed that in order to detect xylene gaswith high sensitivity and high selectivity, it was preferable thatCoCr₂₀₄ phase was predominant and Cr₂O₃ was discontinuously added. Inthis embodiment, the results were shown that the most appropriateconcentration ratio of Cr/Co was 3.0. This was thought to reduce thesensitivity to xylene and all hindered gases when Cr₂O₃ was added inexcess. In addition, it could be confirmed that the selectivity toxylene was lowered at 350° C. or more. Furthermore, it was thought thatthe gas sensor according to an embodiment of the inventive concept wascapable of having the selectivity to xylene in a temperature range of250° C. to 350° C. considering that the oxide semiconductor gas sensorwas capable of detecting the gas at a temperature higher than 250° C.,normally.

FIG. 8 is a graph illustrating selectivity (S_(X)/S_(E)) of xylene inExample 2-2, Example 2-3, and Example 2-4 at an operating temperature275° C. Referring to FIG. 8, the xylene selectivity (S_(X)/S_(E)) ofExamples 2-2, 2-3, and 2-4 was confirmed at the gas sensor operatingtemperature of 275° C. As a result of measurement, it was confirmed thatExample 2-2 had a very good selectivity of xylene to ethanol(S_(X)/S_(E)=87.8). Due to the addition of Pt known as an ethanoloxidation catalyst, ethanol was all oxidized at the upper end of thesensor sensitive layer but all xylene was not decomposed at the upperend of the sensitive layer to diffuse to a lower end of the sensitivelayer. It was thought that as xylene diffused into the sensitivematerial, was converted into a highly reactive gas by Pt and CoCr₂O₄,and promoted a gas sensitive reaction, the addition of Pt decreased thesensitivity of ethanol and greatly increased the sensitivity of xyleneto have high sensitivity and selectivity to xylene. On the other hand,the xylene selectivity (S_(X)/S_(E)=31.0) of Example 2-3 having Pd-addCoCr₂O₄ and the xylene selectivity (S_(X)/S_(E)=25.4) of Example 2-4having Au-add CoCr₂O₄ showed similar or lower values to the xyleneselectivity of Example 2-1 having pure CoCr₂O₄. It was shown that inPd-add CoCr₂O₄, ethanol and some xylene were oxidized and decomposed inthe upper end of the sensor sensitive layer by excessively highcatalytic activity of Pd, to decrease both sensitivity of xylene andethanol similar to the xylene selectivity of pure CoCr₂O₄. It was shownthat in Au-add CoCr₂O₄, the sensitivity of the gases was all increasedby the electrical sensitization, but the sensitivity to ethanol washighly increased and the xylene selectivity was low compared to pureCoCr₂O₄.

FIG. 9 is a graph illustrating sensitivity characteristics of xylene gasin Example 1-2 (300° C.) and Example 2-2 (275° C.) at sensor operatingtemperatures showing the highest selectivity with respect to changes ofgas concentration. Referring to FIG. 9, it was confirmed that Example1-2 showed xylene sensitivity values of 2.19, 3.28, 6.70, 34.65 and109.5 to xylene concentrations of 0.25, 0.5, 1, 2.5 and 5 ppm,respectively, at the operating temperature of 300° C. This was a resultshowing that Example 1-2 was an excellent gas sensor which was capableof being measured with high sensitivity of xylene at a concentrationlevel of 250 ppb. In addition, it was confirmed that Example 2-2 showedxylene sensitivity values of 1.33, 1.58, 1.97, 2.08, and 2.22 to xyleneconcentrations of 25, 50, 75, 100, and 125 ppb, respectively, at theoperating temperature of 275° C. It was shown that the sensor of Example2-2 was capable of measuring trace amounts of xylene at the level of 10ppb. Therefore, the gas sensor according to an embodiment of theinventive concept is capable of measuring the concentration of the traceamount of xylene and a change amount of xylene in real time.

FIG. 10 is a graph illustrating results of comparing xylene sensitivityfor each concentration with the existing studies. Referring to FIG. 10,it was confirmed that each xylene sensitivity of Examples 1-2 and 2-1 ofthe inventive concept was much higher than that of studies (6, 7, 8, 9,10, 11, 12, 13) previously conducted. In particular, it was shown thatthe sensitivity and selectivity to xylene in Example 2-2 weresignificantly superior to the existing results (10, 11, 12, 13). Inaddition, it was confirmed that low concentration of xylene was capableof being effectively detected compared with NiO, Co₃O₄ and the like,which were generally known to have high methylbenzene sensitivity. Aconventional oxide semiconductor-based xylene sensor had very poorsensitivity and selectivity and was difficult to measure only xylene,while the sensor of the inventive concept may measure extremely smallamounts of xylene with high sensitivity and high selectivity.

FIG. 11 is a graph showing results of measuring gas sensitivity (R_(g)R_(a) ⁻¹) of Example 1-2 of the inventive concept for 30 days. Referringto FIG. 11, it was confirmed that the gas sensor according to anembodiment of the inventive concept was capable of maintaining stablegas sensitivity for at least 30 days.

The gas sensor according to an embodiment of the inventive concept mayhave a high selectivity and high sensitivity to xylene.

The above detailed description illustrates the inventive concept. Inaddition, the above-mentioned contents show and explain preferredembodiments of the inventive concept and the inventive concept may beused in various other combinations, modifications, and environments.That is, changes or modifications may be made within the scope of theconcept of the inventive concept disclosed in the present specification,the scope equivalent to the disclosures described above, and/or theskill or knowledge in the art. The described embodiments illustrate thebest state for implementing the technical idea of the inventive conceptand various modifications required in the specific application field anduse of the inventive concept are possible. Thus, the detaileddescription of the inventive concept is not intended to limit theinventive concept to the disclosed embodiments. Also, the appendedclaims should be construed to include other embodiments.

What is claimed is:
 1. A method of manufacturing a gas sensor, themethod comprising: reacting a mixed material including a first materialcontaining a cobalt (Co) element and a second material containing achromium (Cr) element to form a CoCr₂O₄ hollow structure having a hollowshape.
 2. The method of claim 1, wherein the mixed material furtherincludes citric acid.
 3. The method of claim 1, wherein the firstmaterial and the second material are provided to the mixed material tobe a molar ratio between the cobalt element and the chromium element of1:2 to 1:4.
 4. The method of claim 1, wherein the mixed material furtherincludes a noble metal catalyst.
 5. The method of claim 4, wherein thenoble metal catalyst includes Pt, Pd, or Au.
 6. The method of claim 1,wherein the first material includes cobalt (II) nitrate hexahydrate(Co(NO₃)₂.6H₂O), and wherein the second material includes chromium (III)nitrate nonahydrate (Cr(NO₃)₃.9H₂O).
 7. The method of claim 1, whereinthe forming of the hollow structure includes: dissolving the mixedmaterial in distilled water to prepare a spray solution; spraying thespray solution and heating the sprayed spray solution to form a CoCr₂O₄precursor; and performing heat treatment of the CoCr₂O₄ precursor. 8.The method of claim 1, further comprising: coating the CoCr₂O₄ hollowstructure prepared in the forming of the hollow structure on aninsulator substrate where an electrode is provided.
 9. A gas sensorcomprising: a sensitive layer sensitive to xylene, wherein the sensitivelayer includes a CoCr₂O₄ hollow structure.
 10. The gas sensor of claim9, wherein the sensitive layer further includes Cr₂O₃.
 11. The gassensor of claim 9, wherein the sensitive layer further includes a noblemetal catalyst.
 12. The gas sensor of claim 9, further comprising: aninsulator substrate formed of an insulator material; and an electrodeconnected to the insulator substrate, wherein the sensitive layer iscoated on the insulator substrate, and wherein the electrode isconnected between the insulator substrate and the sensitive layer. 13.The gas sensor of claim 9, further comprising: a heater heating thesensitive layer.